Method for compensating for time dispersion in a communication system

ABSTRACT

A method for correcting the adverse affects of time dispersion in a communication system is disclosed. A mobile station is assigned a first rate of transmission. The mobile station can then detects time dispersion in transmissions. The mobile station can then request a lower rate of transmission when the time dispersion can not be compensated by a compensator in the mobile station within a predetermined period of time. In response to the request, the system can send an indication of a second rate of transmission, which is lower than the first rate of transmission, to the mobile station. The mobile station also decreases the complexity of the compensator.

BACKGROUND

Applicants' invention relates to electrical telecommunication, and moreparticularly to wireless communication systems, such as cellular andsatellite radio systems, for various modes of operation (analog,digital, dual mode, etc.), and access techniques such as frequencydivision multiple access (FDMA), time divisional multiple access (TDMA),code divisional multiple access (CDMA), hybrid FDMA/TDMA/CDMA, forexample. The specific aspects of the present invention are directed totechniques for enhancing bandwidth allocation, traffic and capacitymanagement, and the throughput and quality of transactions.

A description follows which is directed to environments in which thesystem of the present invention may be applied. This general descriptionis intended to provide a general overview of known systems and theterminology associated therewith so that a better understanding of theinvention can be achieved. In North America, digital communication andmultiple access techniques such as TDMA are currently provided by adigital cellular radiotelephone system called the digital advancedmobile phone service (D-AMPS), some of the characteristics of which arespecified in the interim standard TIA/EIA/IS-54-B, "Dual-Mode MobileStation-Base Station Compatibility Standard", published by theTelecommunications Industry Association and Electronic IndustriesAssociation (TIA/EIA), which is expressly incorporated herein byreference. Because of a large existing consumer base of equipmentoperating only in the analog domain with frequency-division multipleaccess (FDMA), TIA/EIA/IS-54-B is a dual-mode (analog and digital)standard, providing for analog compatibility together with digitalcommunication capability. For example, the TIA/EIA/IS-54-B standardprovides for both FDMA analog voice channels (AVC) and TDMA digitaltraffic channels (DTC). The AVCs and DTCs are implemented by frequencymodulating radio carrier signals, which have frequencies near 800megahertz (MHz) such that each radio channel has a spectral width of 30kilohertz (KHz).

In a TDMA cellular radiotelephone system, each radio channel is dividedinto a series of time slots, each of which contains a burst ofinformation from a data source, e.g., a digitally encoded portion of avoice conversation. The time slots are grouped into successive TDMAframes having a predetermined duration. The number of time slots in eachTDMA frame is related to the number of different users that cansimultaneously share the radio channel. If each slot in a TDMA frame isassigned to a different user, the duration of a TDMA frame is theminimum amount of time between successive time slots assigned to thesame user.

The successive time slots assigned to the same user, which are usuallynot consecutive time slots on the radio carrier, constitute the user'sdigital traffic channel, which may be considered a logical channelassigned to the user. As described in more detail below, digital controlchannels (DCCs) can also be provided for communicating control signals,and such a DCC is a logical channel formed by a succession of usuallynon-consecutive time slots on the radio carrier.

In only one of many possible embodiments of a TDMA system as describedabove, the TIA/EIA/IS-54-B standard provided that each TDMA frameconsists of six consecutive time slots and has a duration of 40milliseconds (msec). Thus, each radio channel can carry from three tosix DTCs (e.g., three to six telephone conversations), depending on thesource rates of the speech coder/decoders (codecs) used to digitallyencode the conversations. Such speech codecs can operate at eitherfull-rate or half-rate. A full-rate DTC requires twice as many timeslots in a given time period as a half-rate DTC, and in TIA/EIA/IS-54-B,each full-rate DTC uses two slots of each TDMA frame, i.e., the firstand fourth, second and fifth, or third and sixth of a TDMA frame's sixslots. Each half-rate DTC uses one time slot of each TDMA frame. Duringeach DTC time slot, 324 bits are transmitted, of which the majorportion, 260 bits, is due to the speech output of the codec, includingbits due to error correction coding of the speech output, and theremaining bits are used for guard times and overhead signalling forpurposes such as synchronization.

It can be seen that the TDMA cellular system operates in abuffer-and-burst, or discontinuous-transmission, mode: each mobilestation transmits (and receives) only during its assigned time slots. Atfull rate, for example, a mobile station might transmit during slot 1,receive during slot 2, idle during slot 3, transmit during slot 4,receive during slot 5, and idle during slot 6, and then repeat the cycleduring succeeding TDMA frames. Therefore, the mobile station, which maybe battery-powered, can be switched off, or sleep, to save power duringthe time slots when it is neither transmitting nor receiving.

In addition to voice or traffic channels, cellular radio communicationsystems also provide paging/access, or control, channels for carryingcall-setup messages between base stations and mobile stations. Accordingto TIA/EIA/IS-54-B, for example, there are twenty-one dedicated analogcontrol channels (ACCs), which have predetermined fixed frequencies fortransmission and reception located near 800 MHz. Since these ACCs arealways found at the same frequencies, they can be readily located andmonitored by the mobile stations.

For example, when in an idle state (i.e., switched on but not making orreceiving a call), a mobile station in a TIA/EIA/IS-54-B system tunes toand then regularly monitors the strongest control channel (generally,the control channel of the cell in which the mobile station is locatedat that moment) and may receive or initiate a call through thecorresponding base station. When moving between cells while in the idlestate, the mobile station will eventually "lose" radio connection on thecontrol channel of the "old" cell and tune to the control channel of the"new" cell. The initial tuning and subsequent re-tuning to controlchannels are both accomplished automatically by scanning all theavailable control channels at their known frequencies to find the "best"control channel. When a control channel with good reception quality isfound, the mobile station remains tuned to this channel until thequality deteriorates again. In this way, mobile stations stay "in touch"with the system.

While in the idle state, a mobile station must monitor the controlchannel for paging messages addressed to it. For example, when anordinary telephone (land-line) subscriber calls a mobile subscriber, thecall is directed from the public switched telephone network (PSTN) to amobile switching center (MSC) that analyzes the dialed number. If thedialed number is validated, the MSC requests some or all of a number ofradio base stations to page the called mobile station by transmittingover their respective control channels paging messages that contain themobile identification number (MIN) of the called mobile station. Eachidle mobile station receiving a paging message compares the received MINwith its own stored MIN. The mobile station with the matching stored MINtransmits a page response over the particular control channel to thebase station, which forwards the page response to the MSC.

Upon receiving the page response, the MSC selects an AVC or a DTCavailable to the base station that received the page response, switcheson a corresponding radio transceiver in that base station, and causesthat base station to send a message via the control channel to thecalled mobile station that instructs the called mobile station to tuneto the selected voice or traffic channel. A through-connection for thecall is established once the mobile station has tuned to the selectedAVC or DTC.

The performance of the system having ACCs that is specified byTIA/EIA/IS54-B has been improved in a system having digital controlchannels (DCCHs) that is specified in TIA/EIA/IS-136, which is expresslyincorporated herein by reference. Using such DCCHs, each TIA/EIA/IS-54-Bradio channel can carry DTCs only, DCCHs only, or a mixture of both DTCsand DCCHs. Within the TIA/EIA/IS-136-B framework, each radio carrierfrequency can have up to three full-rate DTCs/DCCHs, or six half-rateDTCs/DCCHs, or any combination in between, for example, one full-rateand four half-rate DTCs/DCCHs.

In general, however, the transmission rate of the DCCH need not coincidewith the half-rate and full-rate specified in TIA/EIA/IS-54-B, and thelength of the DCCH slots may not be uniform and may not coincide withthe length of the DTC slots. The DCCiH may be defined on anTIA/EIA/IS-54-B radio channel and may consist, for example, of everyn-th slot in the stream of consecutive TDMA slots. In this case, thelength of each DCCH slot may or may not be equal to 6.67 msec, which isthe length of a DTC slot according to TIA/EIA/IS-54-B. Alternatively(and without limitation on other possible alternatives), these DCCHslots may be defined in other ways known to one skilled in the art.

In cellular telephone systems, an air link protocol is required in orderto allow a mobile station to communicate with the base stations and MSC.The communications link protocol is used to initiate and to receivecellular telephone calls. The communications link protocol is commonlyreferred to within the communications industry as a Layer 2 protocol,and its functionality includes the delimiting, or framing, of Layer 3messages. These Layer 3 messages may be sent between communicating Layer3 peer entities residing within mobile stations and cellular switchingsystems. The physical layer (Layer 1) defines the parameters of thephysical communications channel, e.g., radio frequency spacing,modulation characteristics, etc. Layer 2 defines the techniquesnecessary for the accurate transmission of information within theconstraints of the physical channel, e.g., error correction anddetection, etc. Layer 3 defines the procedures for reception andprocessing of information transmitted over the physical channel.

Communications between mobile stations and the cellular switching system(the base stations and the MSC) can be described in general withreference to FIGS. 1 and 2. FIG. 1 schematically illustrates pluralitiesof Layer 3 messages 11, Layer 2 frames 13, and Layer 1 channel bursts,or time slots, 15. In FlG. 1, each group of channel bursts correspondingto each Layer 3 message may constitute a logical channel, and asdescribed above, the channel bursts for a given Layer 3 message wouldusually not be consecutive slots on an TIA/EIA/136 carrier. On the otherhand, the channel bursts could be consecutive; as soon as one time slotends, the next time slot could begin.

Each Layer 1 channel burst 15 contains a complete Layer 2 frame as wellas other information such as, for example, error correction informationand other overhead information used for Layer 1 operation. Each Layer 2frame contains at least a portion of a Layer 3 message as well asoverhead information used for Layer 2 operation. Although not indicatedin FIG. 1, each Layer 3 message would include various informationelements that can be considered the payload of the message, a headerportion for identifying the respective message's type, and possiblypadding.

Each Layer 1 burst and each Layer 2 frame is divided into a plurality ofdifferent fields. In particular, a limited-length DATA field in eachLayer 2 frame contains the Layer 3 message 11. Since Layer 3 messageshave variable lengths depending upon the amount of information containedin the Layer 3 message, a plurality of Layer 2 frames may be needed fortransmission of a single Layer 3 message. As a result, a plurality ofLayer 1 channel bursts may also be needed to transmit the entire Layer 3message as there is a one-to-one correspondence between channel burstsand Layer 2 frames.

As noted above, when more than one channel burst is required to send aLayer 3 message, the several bursts are not usually consecutive burstson the radio channel. Moreover, the several bursts are not even usuallysuccessive bursts devoted to the particular logical channel used forcarrying the Layer 3 message. Since time is required to receive,process, and react to each received burst, the bursts required fortransmission of a Layer 3 message are usually sent in a staggeredformat, as schematically illustrated in FIG. 2(a) and as described abovein connection with the TIA/EIA/IS-136 standard.

FIG. 2(a) shows a general example of a forward (or downlink) DCCHconfigured as a succession of time slots 1, 2 , . . . , N, . . .included in the consecutive time slots 1, 2, . . . sent on a carrierfrequency. These DCCH slots may be defined on a radio channel such asthat specified by TIA/EIA/IS-136, and may consist, as seen in FIG. 2(a)for example, of every n-th slot in a series of consecutive slots. EachDCCH slot has a duration that may or may not be 6.67 msec, which is thelength of a DTC slot according to the TIA/EIA/IS-136 standard.

As shown in FIG. 2(a), the DCCH slots may be organized into superframes(SF), and each superframe includes a number of logical channels thatcarry different kinds of information. One or more DCCH slots may beallocated to each logical channel in the superframe. The exemplarydownlink superframe in FIG. 2(a) includes three logical channels: abroadcast control channel (BCCH) including six successive slots foroverhead messages; a paging channel (PCH) including one slot for pagingmessages; and an access response channel (ARCH) including one slot forchannel assignment and other messages. The remaining time slots in theexemplary superframe of FIG. 2(a) may be dedicated to other logicalchannels, such as additional paging channels PCH or other channels.Since the number of mobile stations is usually much greater than thenumber of slots in the superframe, each paging slot is used for pagingseveral mobile stations that share some unique characteristic, e.g., thelast digit of the MIN.

FIG. 2(b) illustrates a preferred information format for the slots of aforward DCCH. The information transferred in each slot comprises aplurality of fields, and FIG. 2(b) indicates the number of bits in eachfield above that field. The bits sent in the SYNC field are used in aconventional way to help ensure accurate reception of the codedsuperframe phase (CSFP) and DATA fields. The SYNC field includes apredetermined bit pattern used by the base stations to find the start ofthe slot. The shared channel feedback (SCF) field is used to control arandom access channel (RACH), which is used by the mobile to requestaccess to the system. The CSFP field conveys a coded superframe phasevalue that enables the mobile stations to find the start of eachsuperframe. This is just one example for the information format in theslots of the forward DCCH.

For purposes of efficient sleep mode operation and fast cell selection,the BCCH may be divided into a number of sub-channels. A BCCH structureis known that allows the mobile station to read a minimnum amount ofinformation when it is switched on (when it locks onto a DCCH) beforebeing able to access the system (place or receive a call). After beingswitched on, an idle mobile station needs to regularly monitor only itsassigned PCH slots (usually one in each superframe); the mobile cansleep during other slots. The ratio of the mobile's time spent readingpaging messages and its time spent asleep is controllable and representsa tradeoff between call-set-up delay and power consumption.

Since each TDMA time slot has a certain fixed information carryingcapacity, each burst typically carries only a portion of a Layer 3message as noted above. In the uplink direction, multiple mobilestations attempt to communicate with the system on a contention basis,while multiple mobile stations listen for Layer 3 messages sent from thesystem in the downlink direction. In known systems, any given Layer 3message must be carried using as many TDMA channel bursts as required tosend the entire Layer 3 message.

Digital control and traffic channels are desirable for reasons, such assupporting longer sleep periods for the mobile units, which results inlonger battery life, for example. Digital traffic channels and digitalcontrol channels have expanded functionality for optimizing systemcapacity and supporting hierarchical cell structures, i.e., structuresof macrocells, microcells, picocells, etc. The term "macrocell"generally refers to a cell having a size comparable to the sizes ofcells in a conventional cellular telephone system (e.g., a radius of atleast about 1 kilometer), and the terms "microcell" and "picocell"generally refer to progressively smaller cells. For example, a microcellmight cover a public indoor or outdoor area, e.g., a convention centeror a busy street, and a picocell might cover an office corridor or afloor of a high-rise building. From a radio coverage perspective,macrocells, microcells, and picocells may be distinct from one anotheror may overlap one another to handle different traffic patterns or radioenvironments.

FIG. 3 is an exemplary hierarchical, or multi-layered, cellular system.An umbrella macrocell 10 represented by a hexagonal shape makes up anoverlying cellular structure. Each umbrella cell may contain anunderlying microcell structure. The umbrella cell 10 includes microcell20 represented by the area enclosed within the dotted line and microcell30 represented by the area enclosed within the dashed line correspondingto areas along city streets, and picocells 40, 50, and 60, which coverindividual floors of a building. The intersection of the two citystreets covered by the microcells 20 and 30 may be an area of densetraffic concentration, and thus might represent a hot spot.

FIG. 4 represents a block diagram of an exemplary cellular mobileradiotelephone system, including an exemplary base station 110 andmobile station 120. The base station includes a control and processingunit 130 which is connected to the MSC 140 which in turn is connected tothe PSTN (not shown). General aspects of such cellular radiotelephonesystems are known in the art, as described by U.S. Pat. No. 5,175,867 toWejke et al., entitled "Neighbor-Assisted Handoff in a CellularCommunication System," which is incorporated in this application byreference.

The base station 110 handles a plurality of voice channels through avoice channel transceiver 150, which is controlled by the control andprocessing unit 130. Also, each base station includes a control channeltransceiver 160, which may be capable of handling more than one controlchannel. The control channel transceiver 160 is controlled by thecontrol and processing unit 130. The control channel transceiver 160broadcasts control information over the control channel of the basestation or cell to mobiles locked to that control channel. It will beunderstood that the transceivers 150 and 160 can be implemented as asingle device, like the voice and control transceiver 170, for use withDCCHs and DTCs that share the same radio carrier frequency.

The mobile station 120 receives the information broadcast on a controlchannel at its voice and control channel transceiver 170. Then, theprocessing unit 180 evaluates the received control channel information,which includes the characteristics of cells that are candidates for themobile station to lock on to, and determines on which cell the mobileshould lock. Advantageously, the received control channel informationnot only includes absolute information concerning the cell with which itis associated, but also contains relative information concerning othercells proximate to the cell with which the control channel isassociated, as described in U.S. Pat. No. 5,353,332 to Raith et al.,entitled "Method and Apparatus for Communication Control in aRadiotelephone System," which is incorporated in this application byreference.

To increase the user's "talk time", i.e., the battery life of the mobilestation, a digital forward control channel (base station to mobilestation) may be provided that can carry the types of messages specifiedfor current analog forward control channels (FOCCs), but in a formatwhich allows an idle mobile station to read overhead messages whenlocking onto the FOCC and thereafter only when the information haschanged; the mobile sleeps at all other times. In such a system, sometypes of messages are broadcast by the base stations more frequentlythan other types, and mobile stations need not read every messagebroadcast.

The systems specified by the TIA/EIA/IS-54-B and TIA/EIA/IS-136standards are circuit-switched technology, which is a type of"connection-oriented" communication that establishes a physical callconnection and maintains that connection for as long as thecommunicating end-systems have data to exchange. The direct connectionof a circuit switch serves as an open pipeline, permitting theend-systems to use the circuit for whatever they deem appropriate. Whilecircuit-switched data communication may be well suited toconstant-bandwidth applications, it is relatively inefficient forlow-bandwidth and "bursty" applications.

Packet-switched technology, which may be connection-oriented (e.g.,X.25) or "connectionless" (e.g., the Internet Protocol, "IP"), does notrequire the set-up and tear-down of a physical connection, which is inmarked contrast to circuit-switched technology. This reduces the datalatency and increases the efficiency of a channel in handling relativelyshort, bursty, or interactive transactions. A connectionlesspacket-switched network distributes the routing functions to multiplerouting sites, thereby avoiding possible traffic bottlenecks that couldoccur when using a central switching hub. Data is "packetized" with theappropriate end-system addressing and then transmitted in independentunits along the data path. Intermediate systems, sometimes called"routers", stationed between the communicating end-systems makedecisions about the most appropriate route to take on a per packetbasis. Routing decisions are based on a number of characteristics,including: least-cost route or cost metric; capacity of the link; numberof packets waiting for transmission; security requirements for the link;and intermediate system (node) operational status.

Packet transmission along a route that takes into consideration pathmetrics, as opposed to a single circuit set up, offers application andcommunications flexibility. It is also how most standard local areanetworks (LANs) and wide area networks (WANs) have evolved in thecorporate environment. Packet switching is appropriate for datacommunications because many of the applications and devices used, suchas keyboard terminals, are interactive and transmit data in bursts.Instead of a channel being idle while a user inputs more data into theterminal or pauses to think about a problem, packet switchinginterleaves multiple transmissions from several terminals onto thechannel.

Packet data provides more network robustness due to path independenceand the routers' ability to select alternative paths in the event ofnetwork node failure. Packet switching, therefore, allows for moreefficient use of the network lines. Packet technology offers the optionof billing the end user based on amount of data transmitted instead ofconnection time. If the end user's application has been designed to makeefficient use of the air link, then the number of packets transmittedwill be minimal. If each individual user's traffic is held to a minimum,then the service provider has effectively increased network capacity.

Packet networks are usually designed and based on industry-wide datastandards such as the open system interface (OSI) model or the TCP/IPprotocol stack. These standards have been developed, whether formally orde facto, for many years, and the applications that use these protocolsare readily available. The main objective of standards-based networks isto achieve interconnectivity with other networks. The Internet istoday's most obvious example of such a standards-based network pursuitof this goal.

Packet networks, like the Internet or a corporate LAN, are integralparts of today's business and communications environments. As mobilecomputing becomes pervasive in these environments, wireless serviceproviders such as those using TIA/EIA/IS-136 are best positioned toprovide access to these networks. Nevertheless, the data servicesprovided by or proposed for cellular systems are generally based on thecircuit-switched mode of operation, using a dedicated radio channel foreach active mobile user.

A few exceptions to data services for cellular systems based on thecircuit-switched mode of operation are described in the followingdocuments, which include the packet data concepts.

U.S. Pat. No. 4,887,265 and "Packet Switching in Digital CellularSystems", Proc. 38th IEEE Vehicular Technology Conf., pp. 414-418 (June1988) describe a cellular system providing shared packet data radiochannels, each one capable of accommodating multiple data calls. Amobile station requesting packet data service is assigned to aparticular packet data channel using essentially regular cellularsignalling. The system may include packet access points (PAPS) forinterfacing with packet data networks. Each packet data radio channel isconnected to one particular PAP and is thus capable of multiplexing datacalls associated with that PAP. Handovers are initiated by the system ina manner that is largely similar to the handover used in the same systemfor voice calls. A new type of handover is added for those situationswhen the capacity of a packet channel is insufficient.

These documents are data-call oriented and based on usingsystem-initiated handover in a similar way as for regular voice calls.Applying these principles for providing general purpose packet dataservices in a TDMA cellular system would result in spectrum-efficiencyand performance disadvantages.

U.S. Pat. No. 4,916,691 describes a new packet mode cellular radiosystem architecture and a new procedure for routing (voice and/or data)packets to a mobile station. Base stations, public switches via trunkinterface units, and a cellular control unit are linked together via aWAN. The routing procedure is based on mobile-station-initiatedhandovers and on adding to the header of any packet transmitted from amobile station (during a call) an identifier of the base station throughwhich the packet passes. In case of an extended period of time betweensubsequent user information packets from a mobile station, the mobilestation may transmit extra control packets for the purpose of conveyingcell location information.

The cellular control unit is primarily involved at call establishment,when it assigns to the call a call control number. It then notifies themobile station of the call control number and the trunk interface unitof the call control number and the identifier of the initial basestation. During a call, packets are then routed directly between thetrunk interface unit and the currently serving base station.

The system described in U.S. Pat. No. 4,916,691 is not directly relatedto the specific problems of providing packet data services in TDMAcellular systems.

"Packet Radio in GSM", European Telecommunications Standards Institute(ETSI) T Doc SMG 4 58/93 (Feb. 12, 1993) and "A General Packet RadioService Proposed for GSM" presented during a seminar entitled "GSM in aFuture Competitive Environment", Helsirki, Finland (Oct. 13, 1993)outline a possible packet access protocol for voice and data in GSM.These documents directly relate to TDMA cellular systems, i.e., GSM, andalthough they outline a possible organization of an optimized sharedpacket data channel, they do not deal with the aspects of integratingpacket data channels in a total system solution.

"Packet Data over GSM Network", T Doc SMG 1 238/93, ETSI (Sept. 28,1993) describes a concept of providing packet data services in GSM basedon first using regular GSM signalling and authentication to establish avirtual channel between a packet mobile station and an "agent" handlingaccess to packet data services. With regular signalling modified forfast channel setup and release, regular traffic channels are then usedfor packet transfer. This document directly relates to TDMA cellularsystems, but since the concept is based on using a "fast switching"version of existing GSM traffic channels, it has disadvantages in termsof spectrum efficiency and packet transfer delays (especially for shortmessages) compared to a concept based on optimized shared packet datachannels.

Cellular Digital Packet Data (CDPD) System Specification, Release 1.0(July 1993), which is expressly incorporated herein by reference,describes a concept for providing packet data services that utilizesavailable radio channels on current Advanced Mobile Phone Service (AMPS)systems, i.e., the North American analog cellular system. CDPD is acomprehensive, open specification endorsed by a group of U.S. cellularoperators. Items covered include external interfaces, air linkinterfaces, services, network architecture, network management, andadministration.

The specified CDPD system is to a large extent based on aninfrastructure that is independent of the existing AMPS infrastructure.Commonalities with AMPS systems are limited to utilization of the sametype of radio frequency channels and the same base station sites (thebase station used by CDPD may be new and CDPD specific) and employmentof a signalling interface for coordinating channel assignments betweenthe two systems.

Routing a packet to a mobile station is based on, first, routing thepacket to a home network node (home Mobile Data Intermediate System,MD-IS) equipped with a home location register (HLR) based on the mobilestation address; then, when necessary, routing the packet to a visited,serving MD-IS based on HLR information; and finally transferring thepacket from the serving MD-IS via the current base station, based on themobile station reporting its cell location to its serving MD-IS.

Although the CDPD System Specification is not directly related to thespecific problems of providing packet data services in TDMA cellularsystems that are addresssed by this application, the network aspects andconcepts described in the CDPD System Specification can be used as abasis for the network aspects needed for an air link protocol inaccordance with this invention.

The CDPD network is designed to be an extension of existing datacommunications networks and the AMPS cellular network. Existingconnectionless network protocols may be used to access the CDPD network.Since the network is always considered to be evolving, it uses an opennetwork design that allows the addition of new network layer protocolswhen appropriate. The CDPD network services and protocols are limited tothe Network Layer of the OSI model and below. Doing so allowsupper-layer protocols and applications development without changing theunderlying CDPD network.

From the mobile subscriber's perspective, the CDPD network is a wirelessmobile extension of traditional networks, both data and voice. By usinga CDPD service provider network's service, the subscriber is ableseamlessly to access data applications, many of which may reside ontraditional data networks. The CDPD system may be viewed as twointerrelated service sets: CDPD network support services and CDPDnetwork services.

CDPD network support services perform duties necessary to maintain andadminister the CDPD network. These services are: accounting server;network management system; message transfer server; and authenticationserver. These services are defined to permit interoperability amongservice providers. As the CDPD network evolves technically beyond itsoriginal AMPS infrastructure, it is anticipated that the supportservices shall remain unchanged. The functions of network supportservices are necessary for any mobile network and are independent ofradio frequency (RF) technology.

CDPD network services are data transfer services that allow subscribersto communicate with data applications. Additionally, one or both ends ofthe data communications may be mobile.

To summarize, there is a need for a system providing general purposepacket data services in D-AMPS cellular systems, based on providingshared packet-data channels optimized for packet data. This applicationis directed to systems and methods that provide the combined advantagesof a connection-oriented network like that specified by theTIA/EIA/IS-136 standard and a connectionless, packet data network.

One important aspect in such systems is the allocation of channels orbandwidth. One example of such channel allocation for IS-136 is mobileassisted channel allocation (MACA). In IS-136, a MACA message isreceived before assigning the traffic channel and is typically sent onthe broadcast control channel (BCCH). For example, procedures used inmaking contention- or reservation-based access attempts may be sent inan access parameter message on the fast BCCH. Examples of such IS-136random access parameters include maximum busy/reserved information,maximum retries information, maximum repetitions information, and amaximum stop counter. Since MACA reports are used before assigningtraffic channels, MACA does not provide any information after the mobilestation accesses the system.

Another important aspect of cellular telephone communication systems isequalization which is used to compensate for irregularities ordeficiencies in the radio medium. An equalizer is primarily used inreceiving circuits for the purpose of reducing the effects of multipathpropagation and, in a cellular system, the effects of relative motionbetween the transmitter and receiver. This is described, for instance,in WO 88/05981, which relates to a TDMA system which includes so-calledadaptive equalization. The setting of the equalizer incorporated in theradio receiver is contingent on synchronizing words that are timemultiplexed with data words transmitted from the radio transmitter. Withthe aid of these synchronizing words, the equalizer can be set so as tocompensate for the dispersion properties of the medium. Radio receiverswhich include equalizers are often used for high symbol ratecommunication (>100 kbit/s), the bit sensitivity of which due tomultipath propagation is greater than the bit sensitivity of lowersymbol rate communication. One disadvantage of using equalizers is thatthey increase a receiver's complexity and power consumption.

The absence of an equalizer affords the advantage of enablingnoncoherent demodulation to be applied, which results in a lower degreeof complexity in the receiver and a lower current consumption. Inaddition, a robust receiver is obtained with rapidly varying radiochannels, due to high vehicle speeds. The disadvantage lies in the factthat the demodulation cannot be carried out with time dispersion, whichconstitute a considerable part of the symbol time.

SUMMARY

When designing a mobile station, a mobile station manufacturer musttrade off between a potential user's desire to communicate at thefastest possible rate and the manufacturer's desire to produce a mobilestation with low complexity, i.e., a mobile station that operates at alow rate. According to one embodiment of this invention, the concerns ofboth the user and the manufacturer can be addressed by a time dispersioncompensator which is capable of communicating at the highest possiblerate when conditions are favorable.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of Applicants' invention will be understoodby reading this description in conjunction with the drawings in which:

FIG. 1 schematically illustrates pluralities of Layer 3 messages, Layer2 frames, and Layer 1 channel bursts, or time slots;

FIG. 2(a) shows a forward DCC configured as a succession of time slotsincluded in the consecutive time slots sent on a carrier frequency;

FIG. 2(b) an example of an IS-136 DCCH field slot format;

FIG. 3 illustrates an exemplary hierarchical, or multi-layered, cellularsystem;

FIG. 4 is a block diagram of an exemplary cellular mobile radiotelephonesystem, including exemplary base station and mobile station;

FIG. 5 illustrates one example of a possible mapping sequence;

FIG. 6 illustrates an example of PDCH reassignment;

FIG. 7 illustrates a conventional RAKE Receiver;

FIG. 8 illustrates a conventional equalizer; and

FIG. 9 illustrates a flow chart according to one embodiment of thepresent invention.

DETAILED DESCRIPTION

To aid in the understanding of the present invention, a description forone possible mapping sequence is illustrated in FIG. 5. It will beapparent to one skilled in the art that the present invention is notlimited to this mapping sequence but also applies to other mappingsequences as well. FIG. 5 shows a dedicated packet digital controlchannel (PDCH) example of how one L3 message is mapped into severalLayer 2 frames, an example of a Layer 2 frame mapping onto a time slot,and an example of time slot mapping onto a PDCH channel. The length ofthe forward packet digital control channel (FPDCH) time slots andreverse packet digital control channel (RPDCH) bursts are fixed. Thereare three forms of RPDCH bursts which have different fixed lengths andthe FPDCH slot and the full-rate PDCH are assumed to be on the physicallayer in FIG. 5. In the present invention, the TDMA frame structure isthe same as for IS-136 DCCH and DTC. In the interest of maximalthroughput when a multi-rate transmission is used (double rate PDCH andtriple rate PDCH), an additional FPDCH slot format is specified.

The digital control channel (DCCH) of IS-136 is used to indicate PDCHoperation. FIG. 6 illustrates the relationship between PDCH's belongingto one cell (or more specifically, having a common mother DCCH) andDCCH's in different cells (more specifically, indicated in the DCCHneighboring list as candidates for DCCH reselection). A mobile stationalways first goes to a DCCH at initial cell selection. On the DCCH, thesupport for PDCH is indicated. If the DCCH indicates support for one ormore dedicated PDCH's, the carrier frequency of one PDCH (beacon PDCH)is provided. The mobile station then registers itself on the beacon PDCHand may be reassigned by the system to another dedicated PDCH.

When designing a mobile station, a mobile station manufacturer musttrade off between a potential user's desire to communicate at thefastest rate possible and the manufacturer's desire to produce a mobilestation with low complexity, i.e., low cost. One problem that allcommunication systems have is that they must deal with time dispersion.Time dispersion, or echoes, occur when a signal is transmitted over anair interface. It is important to note that time dispersion occursregardless of the rate of transmission, i.e., the number of timeslots orcodes. Thus, in this context, rate is not defined to be the gross bitrate of the air interface, but rather the number of timeslots assignedto a mobile station in a TDMA system and the number of codes assigned toa mobile station in a CDMA system. As a result, a mobile station or basestation cannot delete the effects of time dispersion by simply reducingthe rate at which a signal is transmitted. Thus, communication systemsneed time dispersion compensators in order to compensate for the timedispersions effects of transmitted signals. A time dispersioncompensator could be an equalizer in TDMA systems or a RAKE receiver inCDMA systems, but is not limited thereto. A diagram of a conventionalRAKE receiver is illustrated in FIG. 7 and a diagram of a conventionalequalizer is illustrated in FIG. 8. A description of the generaloperations of a RAKE receiver and an equalizer are known in the art andthus will not be described herein.

Given the trade-off between a user's desire to communicate at thefastest possible rate and the manufacturer's desire to produce a mobilestation with low complexity, a manufacturer may decide to design amobile station with a time dispersion compensator which is capable ofworking at the maximum possible rate. In such a design, the mobilestation will be complex and as a result, expensive. In the alternative,the time dispersion compensator could be designed to only work at a lowrate resulting in a less complex mobile station only being able tocommunicate at the low rate. However, the present invention allowscommunication at the high rate when the conditions are favorable asillustrated in FIG. 9. For example, a mobile station may be operating ina system at a maximum rate when conditions are favorable. Whenconditions deteriorate, the mobile station will begin experiencing timedispersion problems which it is unable to compensate for within a givenperiod of time. In response to the detected time dispersion problems,the mobile station may send a request to the system asking for a lowerrate. The mobile station then waits for an indication by a base stationmessage as to what rate has been selected. Once the mobile station hasreceived the indication, the mobile station begins operating at the newrate. While the time dispersion problems will also be experienced on thelower rate, the complexity of the time dispersion compensation can bedecreased by using less taps in a RAKE receiver or less states or otherinformation in an equalizer, so that the compensation can be performedmore quickly.

The mobile station can also request a higher rate. For example, in GSM,an equalizer must be able to handle a full-rate channel of 16microseconds. The complexity of the equalizer is dependent on at leastthe amount of time dispersion being encountered and the number ofsymbols per time unit being processed. Thus, if the amount of timedispersion is less than 16 microseconds, then the number of symbols pertime unit being processed can be increased without changing the basiccomplexity of the equalizer. For example, if a mobile operating infull-rate determines that the amount of time dispersion is less than 8microseconds, then the number of symbols being processed by the timedispersion compensation can typically be doubled. In such a situation,the compensator can be programmed to use less taps, states, etc., so asto more quickly perform the time dispersion compensation. As a result,the full-rate mobile station can now operate on a double-rate channel.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for correcting the adverse affects oftime dispersion in a communication system, comprising the stepsof:assigning a mobile station a first number of timeslots or number ofcodes for transmission; detecting at said mobile station time dispersionin transmissions; sending a request, for a lower number of timeslots ornumber of codes for transmission, from said mobile station when saidtime dispersion can not be compensated by a compensator in said mobilestation within a predetermined period of time; sending an indication ofa second number of timeslots or number of codes for transmission, whichis lower than said first number of timeslots or number of codes fortransmission, to said mobile station; and compensating detected timedispersion based on said second number of timeslots or number of codesfor transmission.
 2. A method according to claim 1, wherein saidcompensator is a RAKE receiver.
 3. A method according to claim 2,wherein a complexity of said compensator is reduced by reducing thenumber of taps used in said RAKE receiver.
 4. A method according toclaim 1, wherein said compensator is an equalizer.
 5. A method accordingto claim 1, wherein said compensator is a Viterbi equalizer.
 6. A methodaccording to claim 5, wherein a complexity of said compensator isreduced by reducing the number of states used in said Viterbi equalizer.7. The method of claim 1, wherein said request for a lower number oftimeslots or number of codes is sent to a base station, and saidindication of a second number of timeslots or number of codes is sentfrom the base station.